What causes this?

This RCA tv has a condition that I have not seen. You tv experts, what causes this round spot on the CRT?

http://www.ebay.com/itm/ANTIQUE-TV-RCA-VICTOR-Late-1940s-Early-1950s-WORKS-/231121668915?pt=Vintage_Electronics_R2&hash=item35cfeccb33

Regards

WC

Could the yoke be slipped off? I know that sounds like a simple solution but if a yoke slips off some seems like it looks similar to this. Its been a few years since I worked on one but looks like it is pulled off the tube a little and twisted a lit
tle as well.

I really like this set. I had wondered about the yoke, but I'm certainly not the brightest bulb in the lamp when it comes to tv. Reckon would that condition be terminal, or something that is like an adjustment? If the yoke has slid on the neck of the tube, does more serious damage likely result? Could this tv have been subjected to rough handling and suffered other damage? Why would cause the yoke to move on the CRT? Just curious here, and I appreciate your response.

Regards

WC

Its at 150 now, would it be worth that? I don't know what would cause it to get luuse unless someone loosened it. But like I said its been may years since I worked on old B&W sets. If its not broke I guess you could slide it back to its position.

Most tv's from this era have yokes that are held onto the neck of the picture tube by a round plastic cover and a clamp. The plastic degrades and crumbles away leaving nothing there to hold the yoke tight against the crt. When the tv is then moved or tilted backwards, the yoke slides back causing this effect. It can be fixed by fabricating a new plastic cover and clamp to hold the yoke tight again so its not terminal and not even electrical. The yoke can be pushed back against the crt and a normal raster will be reproduced but as soon as you move the tv the yoke slides back again and is why you will need a new plastic cover and clamp to fix it.

I agree with Tony on this. Very common problem for plastic to degrade on defelction yokes. I've seen it go bad enough to afctyually short out the yoke, but, this one appears to not have gone that far, yet.

what Tony V said